For example, JP-H02-229726-A discloses the method for producing an optical fiber preform in which a glass-fine-particle deposited body prepared by the VAD method is impregnated with a solution composed of one or more of ions of elements selected from the group consisting of B, Ge, P, Sb, Al and Ti or oxide ions of the elements and a solvent and dried, and subsequently heating at high temperature is performed to obtain a transparent glass preform. According to JP-H02-229726-A, the porosity of the glass-fine-particle deposited body desirably falls within the range of 54.5% to 86.4%. There is also described that, in the case where the porosity is smaller than 54.5%, the solution is not absorbed since the deposited body is hard, and that, in the case where the porosity is larger than 86.4%, the deposited body tends to be cracked at the time of the impregnation with the liquid since the deposited body is soft.
JP-2004-345902-A discloses the method for producing an optical fiber preform in which a glass-fine-particle deposited body prepared by the VAD method or the OVD method is heated under a fluorine compound-containing atmosphere to add fluorine thereto and subsequently further heated at high temperature under a fluorine compound-containing atmosphere or an inert gas atmosphere to obtain a transparent glass preform. According to JP-2004-345902-A, it is effective to control the porosity at the start of heating under the fluorine compound-containing atmosphere to 70% or more, in order to homogenize the fluorine content in the finally obtained transparent glass preform. There is also described that, when the porosity is less than 70%, fluorine cannot be sufficiently diffused into the glass-fine-particle deposited body and a sufficient fluorine content is not obtained or an extremely long period of time is required for achieving homogeneous fluorine concentration.
A porous glass body produced by the OVD method or the MMD method in which a target on which glass fine particles are deposited and a burner is relatively reciprocated has a multi-layer structure in which plural layers are stacked. In the case where porosity is too low in a part or all part of the porous glass body having the multi-layer structure, voids between the glass fine particles become very small. Therefore, in the heating and sintering step in which the porous glass body is heated to form a transparent glass preform, the diffusion coefficient of the atmospheric gas such as helium (He) decreases and gas permeability lowers in the porous glass body, so that shrinkage and disappearance of closed pores generated by solidification (neck formation) of the glass fine particles do not sufficiently proceed and residual air bubbles are prone to be generated. On the other hand, in the case where the porosity is too high in a part or all part of the porous glass body, the number of the glass fine particles per unit volume is small and much space is present in the high porosity part, so that the bonding force between the glass fine particles becomes weak and there is a possibility that the deposited body is cracked in the middle of the deposition step of the glass fine particles.
Moreover, when the porosity is greatly varied at a boundary of individual layers, not only the residual air bubbles are prone to be generated at a part having low porosity but also the composition becomes heterogeneous in a radial direction, so that a decrease in quality is invited.